In the manufacture of integrated circuits, photolithography techniques are used to form integrated circuit patterns on a substrate. Typically, the substrate is coated with a photoresist, portions of which are exposed to ultraviolet (UV) radiation through a mask to image a desired circuit pattern on the photoresist. The portions of the photoresist left unexposed to the UV radiation are removed by a processing solution, leaving only the exposed portions on the substrate. These remaining exposed portions are baked during a photostabilization process to enable the photoresist to withstand subsequent processing.
After such processing, in which the integrated circuit components are formed, it is generally necessary to remove the baked photoresist from the wafer. In addition, residue that has been introduced on the substrate surface through processes such as etching must be removed. Typically, the photoresist is "ashed" or "burned" and the ashed or burned photoresist, along with the residue, is "stripped" or "cleaned" from the surface of the substrate.
One manner of removing photoresist and residues is by directing a microwave-energized plasma at the substrate surface. In a photoresist ashing process, the substrate is rapidly heated to a preset temperature by infrared radiation. During the ashing process, exothermic reactions on the substrate surface, variations in direction of heat flow, and changing thermal radiation characteristics of the substrate can result in continuous thermal transients and temperature gradients on the substrate surface. Temperature gradients are often most pronounced during the ramping phase wherein the substrate temperature is raised to the desired level, although lesser gradients may remain when the substrate is maintained in the steady state phase at this desired level. Such thermal transients and temperature gradients are undesirable in an ashing process as non-uniform heating of the substrate typically results in non-uniform ashing of the photoresist.
One approach to uniformly heating a substrate is to use a heating configuration utilizing a plurality of heating zones, the output radiation of each of which is directed to a specific zone of the substrate by a reflector. In such a case the shape of the reflector is not critically important because the heating zones are individually controlled, using separate feedback (e.g., an optical pyrometer output signal) from each zone to aid in determining the zone power control signal. However, in addition to the complexity and cost of using a plurality of pyrometers, such a system typically experiences a slower ramp rate than what can be maximally achieved with full power applied to each zone. In addition, such a system is not effective in low temperature processes because optical pyrometers typically do not work well under 200.degree. C.
Thus, it is an object of the present invention to provide a system for providing uniform heating of a substrate utilizing only a single temperature feedback device. It is further object of the present invention to provide such a system that controls temperatures under low temperature (under 200.degree. C.) process conditions. It is still a further object of the present invention to provide such a system wherein the combination of zone control and the shape of the heating reflector is used to achieve a maximum ramp rate, followed by a steady state phase, wherein the point-to-point temperature gradient on a heated substrate is no greater than 2% of the temperature setpoint. Yet further objects of the invention are to reduce the number of heating sources and the peak power requirements of such a system in order to reduce the size of required switchgear, and thus the area required for implementing the system.